Interpretive Summary: n 1982 Brazil started its first uranium production center in Pocos de Caldas in the state of Minas Gerais. After 13 years of intermittent operation, the mining activities were suspended in 1995. Uranium was extracted by open pit mining. While several hydrogeologic and geochemical studies of the site had been carried out, a long-term plan for remediation of the site was never implemented. This includes studies of acid mine drainage from the waste rock deposited near the site. Accurate estimates of the release rates of metals and radiocuclides and their transport into the subsurface environment are critical to assessing their impact on the environment and developing effective remediaiton schemes. In this study we used the HYDRUS-2D and STEADQL-v4 numerical codes to simulate water flow and geochemical processes, respectively, in one of the waste rock piles at the site. This paper describes the approach used to obtain detailed conceptual representations of the flow regime at the site, and the many geochemical reactions thought to be present. The estimated seepage flow rate from the site was found to be in relatively good agreement with measured data. Preferential flow of water through the fractured rock waste site was shown to have a major effect on the predictions. The separate fluid flow and geochemical calculations provided considerable insight in the processes controlling acid rock drainage, and should serve as an initial step to application of more realistic coupled models.

Technical Abstract:
This paper discusses the use of two numerical models (HYDRUS-2D and STEADQL-v4) for simulating water flow and relevant geochemical processes in one of the waste rock piles of the first uranium mine in Brazil, in order to facilitate the selection of appropriate remediation strategies. The long time scale required for the oxidation of sulfidic wastes (at least 600 years) implies the need to implement permanent remediation actions. The best remediation scheme should depend on the water flow regime inside the waste pile and on the geochemical processes that occur as a result of the interactions between water and the waste (especially oxidative dissolution of pyrite). Accurate modeling of the waste site, which contains a wide range of grain and rock sizes at different degrees of water saturation and is subject to reactive multicomponent transport, entails considerable physical, mathematical and numerical challenges. This paper describes the approach used to obtain a detailed representation of the system involving both unsaturated/saturated flow (most of the physical properties of the waste were estimated from measured data) and the geochemical network reactions (including equilibrium and kinetics reactions).